PARKLAND, Fla. — The 19-year-old accused of killing 17 people at his former high school told police after his arrest that he carried out the mass shooting, authorities said Thursday, apparently acknowledging his role in one of the nation’s worst school shootings. But police did not indicate what might have motivated Nikolas Cruz to shoot students and teachers in their classrooms on Wednesday, and investigators continued to delve into his troubled, violent life and the red flags that littered his path back to Marjory Stoneman Douglas High School.

As Cruz remained held without bond on murder charges, authorities scoured the high school where they say he aimed his AR-15 assault-style rifle and fired round after round into classroom after classroom, leaving a trail of blood and agony before trying to elude police by hiding among those running to escape the carnage.

On the first full day after the school was transformed into a war zone, this idyllic suburb north of Fort Lauderdale grappled with a massacre that added its name to the ever-growing roster of places synonymous with tragedies in public spaces: Columbine. Newtown. Aurora. Charleston. Las Vegas. Sutherland Springs. And, now, Parkland.

“This community is hurting right now,” Broward County Sheriff Scott Israel said Thursday. “Today’s a day of healing. Today’s a day of mourning.”

The country mourned with Parkland, living through what has become a grim routine. Names slowly emerged on Thursday, revealing that the bullets cut down victims apparently indiscriminately, including a student who had recently gotten into the state’s flagship college, a senior who had just gained U.S. citizenship and a football coach who was working at his alma mater. Those killed ranged in age from 14 to 49, and the dead included nine males and eight females, police said. Most were teenagers, just one of them old enough to vote. Three were staffers.

The familiar response played out as the shooting again cracked open fissures in American politics and culture about guns, a debate that seems ever more entrenched. President Trump, in brief remarks at the White House, echoed Florida Gov. Rick Scott (R) in emphasizing a response focused on mental health, eschewing the calls for stricter gun control that follow each attack. In this case, some of the loudest pleas came from students who survived the attack as well as grieving parents.

“President Trump, please do something!” Lori Alhadeff, who lost her daughter Alyssa, said in emotional remarks broadcast on CNN. “Do something. Action! We need it now! These kids need safety now!”

In Florida, Cruz’s past revealed a pattern of disciplinary issues and unnerving behavior. People who knew him said that for years, Cruz had a habit of attacking animals like squirrels and chickens. When he got older, he became isolated, angry and withdrawn, losing his parents and eventually moving into friends’ houses.

At Douglas, his problems began with suspensions. A teacher said administrators had sent out a message suggesting they keep an eye on him. Cruz was expelled last year. On Wednesday, he came back.

New details about the shooting emerged Thursday in court documents and from police officials. Cruz took an Uber to the school, police said in a probable-cause affidavit, wearing a black backpack and carrying a black duffel bag. A staffer recognized him and radioed a co-worker to say that Cruz was approaching. Within a minute, he heard gunshots and called a “Code Red,” which announced an emergency.

Police say Cruz began firing into a series of rooms, returning to two of them as he continued pumping round after round at the huddled, terrified teachers and students. Cruz then went up the stairs, allegedly firing at another room as he traveled through the school building.

As students began to flee the carnage, police say Cruz dumped his rifle and bag of extra ammunition and joined “others who were fleeing and tried to mix in with the group … fearing for their lives,” Israel, the sheriff, said Thursday. Cruz went to a Walmart, bought a drink, sat at a McDonald’s, and eventually left on foot.

Michael Leonard, a police officer from nearby Coconut Creek who came to assist with the police response, spotted Cruz walking down a residential street not far from the school.

“He looked like a typical high school student,” Leonard said Thursday.

A day after being arrested, Cruz made his first court appearance, facing 17 counts of premeditated murder. He mostly looked down at his hands and answered the judge in a low voice. His attorneys did not specifically say Thursday that he had confessed to the shooting, nor did they explicitly deny his involvement, describing him as a “broken young man “ who is “very saddened” by what happened.

“This is an emotionally broken young man,” Gordon Weekes, the public defender, told reporters, adding that Cruz was on suicide watch. “He has been through a lot of trauma. He has suffered significant mental illness, and significant mental trauma.”

Israel, the sheriff, vowed that authorities would make sure that “justice is served.” In addition to those killed, more than a dozen people were wounded in the attack, and some remained in critical condition Thursday, hospital officials said.

Investigators already have interviewed more than 2,000 people as part of the probe and hope to speak to unnamed others who “might enlighten us as to why he did what he did,” Israel said, though he emphasized that a day into the investigation, police did not believe Cruz had any accomplices.

Cruz bought the AR-15 himself legally in Coral Springs, Fla., officials said. So far, it is the only gun authorities have recovered as part of the investigation, said Peter J. Forcelli, special agent in charge of the Miami field division of the Bureau of Alcohol, Tobacco, Firearms and Explosives. The court filing Thursday said Cruz bought the gun about a year before the rampage.

Federal authorities were facing questions about whether they had missed a chance to encounter the gunman before. The FBI was contacted last fall about a comment left on YouTube that mentioned becoming “a professional school shooter.” The YouTube user’s name was Nikolas Cruz. FBI agents spoke to the person who submitted the tip, searched law enforcement databases and were unable to determine who posted the comment, said Robert F. Lasky, special agent in charge of the FBI’s Miami division. Officials now believe Cruz posted that message.

FBI investigators looking into the shooting were pursuing information Thursday suggesting that Cruz might have been associated with a Florida-based white supremacist group, according to a law enforcement official familiar with the probe. But agents were still trying to determine the extent of his involvement with the group, if any, according to the official, who asked not to be identified discussing an ongoing investigation. A spokesman for the group told the Anti-Defamation League that Cruz had participated in some of its training exercises; the group could not be reached for further comment.

While investigators sought to figure out Cruz’s movements and actions leading up to his arrival at the school, others struggled with what he left behind in his wake. At Broward Health North, where some of the injured were being treated, police escorted a tearful group of young people wearing Stoneman Douglas High School T-shirts into the hospital. False alarms about shootings rattled nerves at another school in Broward and at a vigil in Parkland for the victims.

Others were left worried about returning to normal life in the coming days. Judy Weiss, whose 17-year-old son Justin goes to Douglas, was uneasy about sending him back to school when it reopens. Her anxieties were compounded because her son is disabled and unable to communicate. Because of that, he is taken outside by nurses before the flood of students leaving at the final bell, which meant he was already arriving at his bus when panicked students fled the gunfire.

“My biggest fear is that if my son had not left 10 minutes early, he would have been stuck in a four-hour lockdown,” she said. “If he didn’t have to leave 10 minutes early, he can’t run, he can’t hide, he can’t crawl under a desk. … It’s just very frightening. My son is a sitting duck.”

Weiss said she and her husband bought a home in Parkland 20 years ago because it was considered so safe.

“The public schools here have the best program for my son,” Weiss said Thursday. “I don’t feel comfortable, but I have no choice.”

Lori RozsaLori Rozsa is a freelance reporter and frequent contributor to The Washington Post. She is a former correspondent for People magazine and a former reporter and bureau chief for The Miami Herald. Follow

Mark BermanMark Berman covers national news for The Washington Post. He has been at The Post since 2007 and previously covered transportation and local news. Follow

Renae MerleRenae Merle covers white-collar crime and Wall Street for The Washington Post. She has also worked for the Wall Street Journal and the Associated Press. Follow

Sign up for email updates from the "Confronting the Caliphate" series.

Content from Allstate This content is paid for by an advertiser and published by WP BrandStudio. The Washington Post newsroom was not involved in the creation of this content. Learn more about WP BrandStudio.